Method for patterning an organic material to concurrently form an insulator and a semiconductor and device formed thereby

ABSTRACT

A method for fabricating an electronic device includes forming a layer of precursor material for forming a semi-conductor material in a cured state and exposing the precursor material to light. The precursor is heated in the presence of the light to form an insulator in areas exposed to light and a semiconductor in areas not exposed to the light. The light is preferably in the visible range. Suitable precursors may include 6,13-dihydro-6,13-(2,3,4,5-tetrachloro-2,4-cyclohexadieno)-pentacene ( 202 ) to form, for example, pentacene ( 204 ) as the semiconductor and 6,13-pentacenequinone ( 206 ) as an insulator. A device made in accordance with the method is also included.

CROSS REFERENCE TO RELATED CASES

Applicants claim the benefit of U.S. Provisional Application Ser. No.60/625,672, filed. Nov. 5, 2004 and International Application NumberPCT/IB2005/053583, filed Nov. 2, 2005.

This disclosure relates to semiconductor technology and moreparticularly, to methods for patterning organic materials to form bothan insulator and a semiconductor in a same precursor at the same time,and devices formed by employing these methods.

Organic electronics may be employed in making flexible active matrixback planes for display applications. These back planes, which employorganic materials, are capable of supporting circuits and electricaldevices, such as shift registers (Sec, e.g., Gelinck, G. H. et al,Nature Mat. (2004), 3, 106, incorporated herein by reference). Theseactive matrices may be combined with an electrophoretic display effectto obtain a flexible display (See, e.g., Huitema, H. E. A. et al., Proc.IDW(2003), 1663-4, incorporated herein by reference). An organicsemi-conductor may be provided for these applications, but the organicsemi-conductor material has to be structured to obtain the desiredperformance of the transistors.

Structuring of the semiconductor is complicated and often entails amultiple step process. In general, the process includes that a precursorof the semi-conductor is applied by spin coating. This precursor is thenconverted to the semiconductor during a heating step. This semiconductoris subsequently structured by applying a resist, illuminating the resistand developing the resist. After this, the semiconductor is removedlocally by a reactive ion etch (RIE) step, and optionally thephotoresist is removed.

An alternative method is described by Afzali et. al. in Adv. Mater.(2003) 15, 2066, incorporated herein by reference. Afzali uses aphotosensitive pentacene precursor. One disadvantage of this method isthat the photoproducts formed negatively affect the transistorperformance.

A method for fabricating an electronic device includes forming a layerof precursor material for forming a semiconductor material in a curedstate and exposing the precursor material to light. The precursor isheated in the presence of the light to form an insulator in areasexposed to light and a semiconductor in areas not exposed to the light.The light is preferably in the visible range. Suitable precursors mayinclude6,13-dihydro-6,13-(2,3,4,5-tetrachloro-2,4-cyclohexadieno)-pentacene toform, for example, pentacene as the semiconductor and6,13-pentacenequinone as an insulator. The precursor may include6,13-dihydro-6,13-(2,3,4,5-tetrabromo-2,4-cyclohexadieno)-pentacene, alarger acene precursor, and/or a nonacene precursor.

These and other objects, features and advantages of the presentdisclosure will become apparent from the following detailed descriptionof illustrative embodiments thereof, which is to be read in connectionwith the accompanying drawings.

This disclosure will present in detail the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a substrate having a precursormaterial formed thereon in accordance with one embodiment of the presentdisclosure;

FIG. 2 is a cross-sectional view of the substrate of FIG. 1 having amask for selective exposure of the precursor material in accordance withone embodiment of the present disclosure;

FIG. 3 is a cross-sectional view of the substrate of FIG. 2 showing theexposure to light and the heat processes of the precursor material inaccordance with one embodiment of the present disclosure;

FIG. 4 is a cross-sectional view of the substrate of FIG. 3 after theformation of insulator portions and semiconductor portions in accordancewith one embodiment of the present disclosure;

FIG. 5 is a top view of an electronic showing an insulator portion and asemiconductor portion in accordance with one experimental embodiment ofthe present disclosure;

FIGS. 6 and 7 show spectrographs demonstrating material characteristicsfor the structure of FIG. 5 in accordance with the present disclosure;

FIG. 8 is a block diagram showing method steps for fabricating asemiconductor device in accordance with the present disclosure;

FIG. 9 is a diagram showing an illustrative chemical structure for aprecursor material and its formation into an insulator and asemiconductor material in accordance with one aspect of the presentdisclosure; and

FIG. 10 is a diagram showing an illustrative chemical structure for analternative precursor material in accordance with one aspect of thepresent disclosure.

The present disclosure describes an efficient way to pattern organicmaterials. In one preferred embodiment, pentacene is patterned.Pentacene is a semiconductor material, which has many advantages forsemiconductor performance. For example, pentacene may be applied fromsolution in the form of a precursor, and then converted to pentacene andstructured using standard photolithography techniques in combinationwith a reactive ion etch (RIE). The RIE etch is particularly cumbersome,yet needed since pentacene is insoluble in practically every practicalsolvent.

The present disclosure makes use of the observation that under lightexposure (at elevated temperatures) oxides, such as,6,13-pentacenequinone are formed from the precursor instead ofpentacene. 6,13-pentacenequinone is not semi-conducting. Thus, a methodis presented in which semiconducting pentacene islands are formedembedded in an amorphous transparent insulator matrix, making thedeposition of additional photoresists and the RIE etch process obsoleteor unnecessary.

Methods for fabricating a semiconductor structure are illustrativelyshown with reference to FIGS. 1-4. Referring now in specific detail tothe drawings in which like reference numerals identify similar oridentical elements throughout the several views, and initially to FIG.1, a layer of precursor pentacene 10 is spun unto a substrate 12. Theprecursor 10 may include6,13-dihydro-6,13-(2,3,4,5-tetrachloro-2,4-cyclohexadieno)-pentacene,and/or6,13-dihydro-6,13-(2,3,4,5-tetrabromo-2,4-cyclohexadieno)-pentacene,which may be custom made by SYNCOM™, in the Netherlands.

Substrate 12 may include a semiconductor material, transparent conductoror other material depending on the application for the completed device.Since the precursor material 10 is preferably in a liquid state,material 10 is easily applied to substrate 12 by spin coating; however,other application processes are also contemplated.

Referring to FIGS. 2 and 3, precursor 10 is heated in the presence ofvisible light; however, UV light or IR light can also be used. Byexposing, precursor 10 to light during a heating step, the conversion ofprecursor pentacene to pentacene is influenced. Light 26 (FIG. 3) may bemasked by a mask 28, to pattern precursor 10 during semiconductorprocessing. This permits semiconductor structures to be formedsimultaneously with insulating structures in a same material beingprocessed.

During this method, precursor pentacene is converted to an oxide orquinone, and, in particular, 6,13-pentacenequinone, which is an oxidizedform of pentacene. 6,13-pentacenequinone is an insulator and not aconducting or semiconducting material. As such, a plurality ofadvantages can be provided as a result of these qualities, as would beapparent to one skilled in the art with knowledge of the presentdisclosure.

Heat sufficient to cause the conversion of precursor 10 in the presenceof oxygen to pentacene may include applying a temperature of above about150 degrees Celsius for a sufficient duration, say for example, at least5 seconds or more, other recipes for conversion may also be employed,for example, 200° C. for about 10 seconds, etc. Higher temperatures mayhave ill effects on other electronic devices or materials. Higher orlower temperatures may be employed and application times adjustedaccording to the temperature selected. Ambient oxygen or even more orless concentrations of oxygen are sufficient to form the oxide.Additionally, low light intensities, for example, bulb light intensities(say about 5-60 Watts) are sufficient to alter the conversion ofprecursor 10 to pentacene. Incident intensities are preferably in therange of between about 0.1 mW/cm² and about 1 W/cm² for white light. Awide band of light from IR through visible to UV may be employed toprovide the conversion.

Referring to FIG. 4, pentacene 20 is formed in areas where light wasabsent and 6,13-pentacenequinone 22 is formed in areas where light waspresent during the heating step. In this way, semiconducting areas 20(pentacene) and insulating areas 22 (6,13-pentacenequinone) aresimultaneously formed in a same layer 11. Pentacene forms apolycrystalline (hence, light scattering) film with a purple color,while 6,13-pentacenequinone is transparent and amorphous and thus doesnot give any contrast. Patterned areas 20 and 22 may be employed to formelectronic components such as transistors, diode, or even horizontallydisposed capacitors. Other devices and components may also be formed inaccordance with this disclosure.

In accordance with the present disclosure, patterning a pentaceneprecursor permits a manufacturer to eliminate resist structuring (andremoval) and etching (e.g., RIE etch), and replace these with a simplepattern-wise illumination during the conversion step (e.g., precursor topentacene). The present inventors have found that an illumination (withwhite light, but also with visible (e.g., yellow light), UV or IR) atmodest light intensities during the temperature (heating) step,influences the conversion of the precursor pentacene to pentacene suchthat in the presence of light, the “normal” retro Diels-Alder (leadingto pentacene) does not take place, but the oxidized form ofpentacene(6,13-pentacenequinone) is formed, which is an insulatorinstead of a (semi-) conductor.

This is demonstrated in FIGS. 5-7 in an experiment conducted by thepresent inventors. Referring to FIG. 5, a pentacene precursor material10 was spun onto an electronic device 40. A given area 42 was exposed towhite light while ramping up the temperature using a hot plate. Inabsence of the light, pentacene is formed in area 44. The pentacene inarea 44 formed a polycrystalline (hence, light scattering) film with apurple color, in this case. The 6,13-pentacenequinone in area 42 formedin the illuminated area and is transparent and amorphous and thus doesnot give any contrast.

Referring to FIG. 6, reflectance versus wave numbers arc shown for thematerial in area 42 (“inside spot”) of FIG. 5. Arrows 50 indicate pointsof interest for 6,13-pentacenequinone (quinone). The spectrumdemonstrates the formation of 6,13-pentacenequinone inside the spot inarea 42.

Referring to FIG. 7, reflectance versus wave numbers are shown for thematerial in area 44 (“outside spot”) of FIG. 5. Arrows 54 indicatepoints of interest for 6,13-pentacenequinone. The spectrum demonstratesthe formation of 6,13-pentacenequinone outside the spot in area 44.

The present disclosure may find utility in many different devices andmanufacturing procedures. One procedure for fabricating polymertransistors may be found in G. H. Gelinck et al, Nature Materials, 3,106-110. This procedure may be followed up to the step in whichprecursor pentacene film is spin coated, as described above.

Alternative methods for making transistors in which pentacene islandsare embedded in an amorphous matrix of the quinone will now be generallydescribed. Referring to FIG. 8, in block 100, a surface or substrate isprepared for the application of a pentacene precursor. This may includethe procedures outlined in G. H. Gelinck et. al., as referenced above.In block 102, the precursor is formed on the surface. This may beperformed by spin coating the surface, although other methods may beemployed depending on the application.

In block 104, the layer of precursor is exposed locally to light througha mask at elevated temperatures. The light is preferably white light oryellow light, however, UV or IR can be used as well. In one embodiment,coherent light from a laser or diode may be preferred for increasing theresolution of the projected image. Other image producing techniques mayalso be employed.

Alternately, in block 106, the precursor material is heated up locallyby a light source (laser, infrared (IR) lamp, etc.), which may exposeand heat portions of the precursor simultaneously, thus forming the6,13-pentacene quinone. This step needs a normal heating step to formpentacene in non-illuminated areas.

In either case, the exposure of the precursor is best applied in anenvironment where oxygen is present such that a quinone is formed. Ineither case, the precursor layer can be patterned down to dimensions ofabout tenths of micrometers or less, e.g., sufficient for use inintegrated circuits and active matrix displays, and in particularflexible displays and polymer electronics devices.

It is to be understood that the present disclosure have been describedin terms of pentacene and its oxides, but the teachings herein may beextended to other compounds and chemicals in the oligocene group, suchas for example,7,16-dihydro-7,16-(2,3,4,5-tetrachloro-2,4-cyclohexadieno)-heptacene,which form oxides 7,16-heptacene quinone in the presence of light.Furthermore, larger acene precursors may be employed. The insulatorsformed may include larger sibling molecules of 6,13-pentacene as well(this depends upon the type of starting precursor used).

A chemical diagram of a preferred embodiment will now be illustrativelydescribed. Referring to FIG. 9, a precursor molecule 202 isillustratively shown forming different states in accordance with thepresent disclosure. Molecule 202 is a halogenated molecule, and thepreferred halogens are chlorine and bromine (e.g., X═Cl and/or Br). Itis not necessary that all substitutions are the same halogen, althoughtetra chloro is preferred. Another structure shown in FIG. 10 may alsobe employed and the same process used as described herein

With reference to FIG. 9, in a first instance, molecule 202 is convertedto pentacene 204 under increased temperature (ΔT). In a second instance,in the presence of heat (ΔT), light (hν) and oxygen (O₂), the precursor202 forms a quinone, such as 6,13-pentacenequinone 206.

Having described preferred embodiments for method for patterning anorganic material and device formed thereby (which are intended to beillustrative and not limiting), it is noted that modifications andvariations can be made by persons skilled in the art in light of theabove teachings. It is therefore to be understood that changes may bemade in the particular embodiments of the disclosure disclosed which arewithin the scope and spirit of the embodiments disclosed herein asoutlined by the appended claims. Having thus described the details andparticularity required by the patent laws, what is claimed and desiredprotected by Letters Patent is set forth in the appended claims.

1. A method for fabricating an electronic device, comprising the stepsof: forming a layer of precursor material for forming a semiconductormaterial in a cured state; and heating the precursor in the presence oflight and ambient oxygen to form an insulator in areas exposed to lightand a semiconductor in areas not exposed to the light, wherein theprecursor is a pentacene precursor, and the pentacene precursor ispatterned in the presence of light and ambient oxygen, thereby formingan oxide of pentacene without performing additional steps of depositingphotoresist and applying a reactive ion etch.
 2. The method as recitedin claim 1, wherein the step of heating includes exposing the precursormaterial to light in the visible, UV and/or IR region.
 3. The method asrecited in claim 2, wherein the light includes white light having anintensity of between about 0.1 mW/cm² and 1 W/cm².
 4. The method asrecited in claim 1, wherein the step of heating includes exposing theprecursor material to light and heat simultaneously using a laser. 5.The method as recited in claim 1, wherein the step of heating theprecursor includes heating the precursor to higher than 150 degreesCelsius.
 6. The method as recited in claim 1, wherein the step ofheating the precursor includes heating the precursor in the presence ofoxygen to form an oxide in areas exposed to the light.
 7. The method asrecited in claim 1, wherein the insulator includes a quinone.
 8. Themethod as recited in claim 7, wherein the quinone includes6,13-pentacenequinone, or a larger sibling material.
 9. The method asrecited in claim 1, wherein the semiconductor material includespentacene.
 10. The method as recited in claim 1, wherein the step ofheating the precursor in the presence of light includes simultaneouslyheating and illuminating the precursor in selected regions with a lightsource and subsequently heating the precursor to form a semiconductor innon-illuminated regions.